Elastomers in principle have found widespread applications in numerous applications. Furtheron a lot of specialty rubbers are available with dispose of a broad range of mechanical, chemical as well as physical properties. Nitrile rubber (NBR) as well as the hydrogenation product thereof, i.e. hydrogenated nitrile rubber, also abbreviated as “HNBR”, represent such specialty rubbers. In particular HNBR has very good heat resistance, an excellent resistance to ozone and chemicals and also an excellent oil resistance. HNBR is used, for example, for seals, hoses, belts and clamping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of oil extraction and also for numerous parts in the aircraft industry, the electronics industry, mechanical engineering and shipbuilding.
However, with the developing of technology, the demands of modern industries for functional rubbery accessories become stricter. It is essential to look for new vulcanizable compounds combining specialty rubbers with additives to improve the properties of elastomeric materials. Since the discovery of carbon nanotubes (CNTs), they have attracted many researchers' attentions owing to their excellent mechanical, electrical and thermal properties. CNTs as reinforcing fillers incorporated into elastomers can improve the mechanical properties of the matrix effectively.
Carbon nanotubes can be viewed as elongated fullerenes (Nature, 1985, Vol. 318, 162). Like fullerenes, carbon nanotubes are made of hexagons, with pentagons only on the ends. Structurally, the shape of a CNT could be imagined that a grapheme sheet rolls into tubule form with end seamless caps together with very high aspect ratios of 1000 or more. As individual molecules, the CNT is believed to be a defect-free structure leading a high strength despite their low density.
There are two basic forms for carbon nanotubes, those produced from a single graphite sheet, referred to as single wall nanotubes (SWNTs), and those nanotubes made up of several concentric sheets known as multi-wall nanotubes (MWNTs). SWNTs have created considerable interest in the academic community with several pertinent reviews on the subject including those by Bahr & Tour (J. Mater. Chem., 2002, 12, 1952), Hirsch (Angewandte Chemie-International Edition, 2002, 41, 1853), Colbert (Plastics Additives & Compounding, January/February 2003, 18) and Baughman & Heer (Science, 2002, 297, 787)
Since carbon nanotubes were discovered more than two decades ago, there have been a variety of techniques developed for producing them. Iijima (Nature, 1991, 354, 56) first observed multi-walled nanotubes. Iijima et al. and Bethune et al. (Nature, 1993, 363, 605) independently reported the synthesis of single-walled nanotubes a few years later. Primary synthesis methods for single and multi-walled carbon nanotubes include arc-discharge (Nature, 1997, 388, 756), laser ablation (Applied Physics A: Materials Science & Processing, 1998, 67, 29), gas-phase catalytic growth from carbon monoxide (Chemical Physics Letters, 1999, 313, 91), and chemical vapor deposition (CVD) from hydrocarbons (Applied Physics Letters, 1999, 75, 1086; Science, 1998, 282, 1105). Subsequent purification steps are required to separate the tubes. The gas-phase processes tend to produce nanotubes with fewer impurities and are more amenable to large-scale processing. Though there are no low-cost, large scale production methods to date, the traditional methods are being developed further and new methods such as fluidized bed reactors are being investigated to create a steady, reasonably priced CNT supply. The low CNT availability and their high prices have limited realization of polymer-CNT composites for many practical applications.
Hydrogenated carboxylated nitrile rubber (also abbreviated as “HXNBR”), prepared by the selective hydrogenation of carboxylated nitrile-conjugated diene rubber (also abbreviated as “XNBR”, being a co-polymer comprising repeating units of at least one conjugated diene, at least one unsaturated nitrile, at least one carboxylated monomer and optionally further comonomers), is a specialty rubber which has very good heat resistance, excellent ozone and chemical resistance, and excellent oil resistance. Coupled with the high level of mechanical properties of the rubber (in particular the high resistance to abrasion) it is not surprising that XNBR and HXNBR have found widespread use in the automotive (seals, hoses, bearing pads) oil (stators, well head seals, valve plates), electrical (cable sheeting), mechanical engineering (wheels, rollers) and shipbuilding (pipe seals, couplings) industries, amongst other industries.
The process for preparation of HXNBR polymers has been described in WO-A-2001/077185 while several other patents applications have been filed relating to various compounding techniques with respect to HXNBR polymers like e.g. WO-A-2005/080493 and WO-A-2005/080492.
Carbon nanotubes, sometimes considered as the “ultimate” fibers, have different and interesting applications. One that has not yet been explored in detail is the question of incorporating the tubes into elastomer materials. Up to now solvent mixing, melt mixing and the spray drying process have been employed as processing methods to prepare some rubber/CNTs composites. The rubber matrixes in the existing studies include natural rubber (NR), styrene butadiene rubber (SBR), chloroprene rubber, silicone rubber, fluorocarbon elastomer (FKM) and hydrogenated acrylonitrile rubber (HNBR).
In Composites Science & Technology, 2003, 63, 1647 the impact of using carbon nanoparticles in silicone based elastomers on the mechanical properties of the resulting specimens is investigated. Using single-wall carbon nanotubes or larger carbon nanofibrils leads to an enhancement of the initial modulus of the resulting specimens as a function of the filler load, however, accompanied by a reduction of the ultimate properties.
The incorporation of carbon nanotubes into polymer matrices has already been explored for a variety of polymers such as siloxanes, isoprene rubber, nitrile butadiene, fluoro polymers (FKM), and hydrogenated nitrile butadiene rubber (HNBR).
In Journal of Material Science, 2006, 41, p. 2541 the effect of MWNTs on curing and mechanical properties of HNBR is described. Two methods are used to prepare the nanocomposites. In the first method CNTs were mixed into HNBR directly on a two roll mill with a curing agent at 50° C. for 10 min, and then the corresponding compound was vulcanized at 170° C. through hot pressing for T90. The second method comprised that low molecular liquid HNBR (LHNBR) was firstly dissolved in acetone, subsequently, the surface modified CNTs were added into the solution, and then the ultrasonic dispersion was used on the mixture. Removing the acetone from the mixture by vacuum drying, a compound with CNTs pre-dispersed in LHNBR was obtained. When using this solvent method the highest tensile strength of the HNBR/MWNT—composites was 18.6 MPa with 25 phr MWNT content.
CN 1554693 discloses the modification of HNBR via carbon nanotubes to enhance the heat-resistance, wearability and mechanical strength of HNBR. To prepare the HNBR composite rubber material carbon nanotubes and liquid rubber are ultrasonically mixed firstly and then added into partially hydrogenated nitrile-butadiene rubber to prepare a masterbatch; this masterbatch is then mixed with the remaining amount of hydrogenated nitrile-butadiene rubber, carbon black, zinc oxide and sulfurizing agent. The mixture is blended on a rolling mixer or a Banbury mixer; and then via vulcanization, the carbon nanotube modified hydrogenated nitrile-butadiene rubber is produced.
US 2006/0061011 teaches the heat conductivity dependence of a polymer-carbon nanotube composite relating to the orientation of the carbon nanotubes. The recommended polymer matrices include styrene butadiene rubber (SBR), nitrile rubber (NBR) and hydrogenated nitrile rubber (HNBR). These polymer-carbon nanotube composites have been used for the manufacture of a pneumatic tire and a wheel for a vehicle.
CA 2,530,471 describes methods for the manufacturing of carbon nanotube-elastomer composites. It is further disclosed that the tensile modulus of such composites is enhanced. As elastomers polysiloxanes, polyisoprene, polybutadiene, polyisobutylene, halogenated polyisoprene, halogenated polybutadiene, halogenated polyisobutylene, low-temperature epoxy, EPDM, polyacrylonitrile, acrylonitrile-butadiene rubber, styrene butadiene rubber, EPM and other alpha-olefine based copolymers, as well as some particular fluorine containing copolymers are mentioned.
JP 2003/322216 teaches the manufacture of a toothed belt in which the surface of the tooth belt comprises a polymer latex, such as styrene butadiene rubber, chloroprene rubber, nitrite rubber and hydrogenated nitrile rubber. These polymer composites are generated through the mixing of carbon nanotubes in the presence of a resorcinol-formaldehyde resin.
In view of the steady demand for elastomeric compounds it is the object of the present invention to provide new vulcanizable compounds combining specialty rubbers with additives. Hydrogenated carboxylated acrylonitrile-butadiene rubber (“HXNBR”) itself already possesses an attractive property profile encompassing oil resistance, abrasion resistance as well as good adhesion to metals. However, due to the particular carboxyl group content HXNBR has not been investigated in such detail as other commodity elastomers and its behaviour in any compound is not foreseeable based on results which might be available for other more typical elastomers. As, however, the applications for which HXNBR may be suited, are extreme ones such as oil well specialties, high performance belts, and roll coverings there is still room for improvement and new HXNBR based compositions.